Proposal: String representation/RleMatrix

Comienza con el ejemplo reproducible en R del issue y revisa cómo se representan los ensayos CollapsedVcf, especialmente las list-matrices y el comportamiento de RleArray/RleMatrix de DelayedArray. La propuesta necesita una representación acordada y un plan de compatibilidad antes de la implementación; el trabajo estaría terminado cuando exista un diseño confirmado que reduzca el uso de memoria sin perder información y que esté validado frente a los workflows afectados.

I'd like to propose a couple of potential optimisations, particularly useful for cases where many entries are the same (e.g. AD: NA NA for GT: "./."). I'm working with a file which compresses nicely to .rds (~40MB) but is very large when in memory (17GB). This size becomes difficult to work with on a local machine or interactively.

Looping over the individual assays in a CompressedVcf I see that the largest ones are those which are matrices of lists (e.g. AD, AF, MBQ, ...). In theory (I think) they contain the same scale of information as GT, but their representation makes them much larger.

I appreciate that this list format makes working with the data cleaner than storing the entries as delimited strings (as I believe they natively are in the VCF) which likely requires parsing the string and splitting into a list-like structure anyway, but this is a tradeoff between size and usability, and applies to the entire dataset even if a user is only interested in a subset.

Would there be interest in a representation of CollapsedVcf list-matrices as either delimited strings (in which case they could be stored as RleMatrix for an additional compression boost; or RleListMatrix (which doesn't exist, but here's an open issue: https://github.com/Bioconductor/DelayedArray/issues/62)? I'm not well-versed enough in the Rle side to know if Rle provides a benefit when stored in a list, but it's an option.

Below is a comparison of object sizes for a toy example 'assay' constructed as a matrix of list elements and the size savings are potentially very large (in this case 1/62 the size).

## create a matrix of list elements
x <- matrix(
  replicate(
    2e4, 
    c(
      sample(c(NA, 1, 2), 1, prob = c(100, 1, 1)), 
      sample(c(NA, 1, 2), 1, prob = c(100, 1, 1))
    ), 
    simplify = FALSE), 
  ncol = 10
)
head(x)
#>      [,1]      [,2]      [,3]      [,4]      [,5]      [,6]      [,7]     
#> [1,] Numeric,2 Numeric,2 Numeric,2 Numeric,2 Numeric,2 Numeric,2 Numeric,2
#> [2,] Numeric,2 Numeric,2 Numeric,2 Numeric,2 Numeric,2 Numeric,2 Numeric,2
#> [3,] Numeric,2 Numeric,2 Numeric,2 Numeric,2 Numeric,2 Numeric,2 Numeric,2
#> [4,] Numeric,2 Numeric,2 Numeric,2 Numeric,2 Numeric,2 Numeric,2 Numeric,2
#> [5,] Numeric,2 Numeric,2 Numeric,2 Numeric,2 Numeric,2 Numeric,2 Numeric,2
#> [6,] Numeric,2 Numeric,2 Numeric,2 Numeric,2 Numeric,2 Numeric,2 Numeric,2
#>      [,8]      [,9]      [,10]    
#> [1,] Numeric,2 Numeric,2 Numeric,2
#> [2,] Numeric,2 Numeric,2 Numeric,2
#> [3,] Numeric,2 Numeric,2 Numeric,2
#> [4,] Numeric,2 Numeric,2 Numeric,2
#> [5,] Numeric,2 Numeric,2 Numeric,2
#> [6,] Numeric,2 Numeric,2 Numeric,2

## collapse to singleton strings (credit: @lawremi)
v <- unlist(x)
v[is.na(v)] <- "."
x_str <- matrix(
  S4Vectors::unstrsplit(
    c(relist(v, x)), ","), 
  nrow(x), ncol(x))
head(x_str)
#>      [,1]  [,2]  [,3]  [,4]  [,5]  [,6]  [,7]  [,8]  [,9]  [,10]
#> [1,] ".,." ".,." ".,." ".,." ".,." ".,." ".,." ".,." ".,." ".,."
#> [2,] ".,." ".,." ".,." ".,." ".,." ".,." ".,." ".,." ".,." ".,."
#> [3,] ".,." ".,." ".,." ".,." ".,." ".,." ".,." ".,." ".,." ".,."
#> [4,] ".,." ".,." ".,." ".,." ".,." ".,." ".,." ".,." ".,." ".,."
#> [5,] ".,." ".,." ".,." ".,." ".,." ".,." ".,." ".,." ".,." ".,."
#> [6,] ".,." ".,." ".,." ".,." ".,." ".,." ".,." ".,." ".,." ".,."

suppressMessages(library(DelayedArray))

## Rle representation of matrix of list doesn't work
as(x, "RleArray")
#> Error in .Call2("Rle_constructor", values, lengths, PACKAGE = "S4Vectors"): Rle of type 'list' is not supported

## Rle representation of string matrix
x_rle <- as(x_str, "RleArray")
x_rle
#> <2000 x 10> matrix of class RleMatrix and type "character":
#>         [,1]  [,2]  [,3]  ... [,9]  [,10]
#> [1,]    ".,." ".,." ".,." .   ".,." ".,."
#> [2,]    ".,." ".,." ".,." .   ".,." ".,."
#> [3,]    ".,." ".,." ".,." .   ".,." ".,."
#> [4,]    ".,." ".,." ".,." .   ".,." ".,."
#> [5,]    ".,." ".,." ".,." .   ".,." ".,."
#> ...     .     .     .     .   .     .    
#> [1996,] ".,." ".,." ".,1" .   ".,." ".,."
#> [1997,] ".,." ".,." ".,." .   "2,." ".,."
#> [1998,] ".,." ".,." ".,." .   ".,." ".,."
#> [1999,] "1,." ".,." "1,." .   ".,." ".,."
#> [2000,] "2,." ".,." ".,." .   ".,." ".,."

## compare sizes
pryr::object_size(x)
#> Registered S3 method overwritten by 'pryr':
#>   method      from
#>   print.bytes Rcpp
#> 1.44 MB
pryr::object_size(x_str)
#> 161 kB
pryr::object_size(x_rle)
#> 23 kB

^{Created on 2020-02-28 by the reprex package (v0.3.0)}

I'm not across this package enough to have any insights into implementation issues or how this might affect other aspects of the inner workings (or user-side workings) but this approach reduces a 1.71GB CollapsedVcf into a 20MB object of the same class without destroying any information. Credit to @lawremi for guidance towards optimising the conversion and on structural advice.